The synthesis of polyethylene isophthalate (hereinafter referred to as PEI) and copolymers thereof has been accompanied by the formation of large amounts of an undesirable crystalline by-product, commonly referred to as ethylene isophthalate cyclic dimer. This compound creates problems during PEI preparation by subliming during the later stages of polymerization when the molten polymer is subjected to vacuum. Cyclic dimer thereafter deposits on the walls of the reaction vessel and plugs vacuum lines. Processing costs are thereby greatly increased due to the necessity of cleaning the vessel and the vacuum lines. Further, the theoretical polymer yield is reduced by an amount corresponding to the amount of cyclic dimer formed which can be as high as 15% as observed in an article by C. E. Berr, J. Polymer Science, 15, 591 (1955). This of course greatly adds to the already considerable raw material costs. Also, the final polymer is contaminated with the high melting cyclic dimer, hence, films or molded articles of PEI contain opaque, white specks of this unmelted by-product. The non-homogeneity of such films and articles severely detracts from their appearance.
Heretofore, polymerizations and copolymerizations involving PEI have been carried out using standard, well-known polycondensation catalysts to increase the rate of polymerization to an acceptable level. Commonly, these catalysts are metallic compounds such as antimony trioxide and various titanium compounds such as alkyl titanates. The present invention utilizes non-metallic, protonic acid catalysts. It has been discovered that the use of non-metallic catalysts yields isophthalate-based homopolymers and copolymers which have reduced cyclic dimer content.
The known art has long recognized the problem of cyclic dimer formation, as for example, C. E. Berr, supra. This article does not, however, teach the use of non-metallic catalysts to reduce cyclic dimer content. U.S. Pat. No. 2,965,613 to Milone, et al, relates to random copolyester of ethylene isophthalate and ethylene terephthalate. While this patent recognizes that cyclic dimerization occurs with polyethylene isophthalate, there is no suggestion of how to reduce the amount of cyclic dimer in the final polymer other than to prepare copolymers containing high amounts of terephthalate units, i.e., greater than about 50% terephthalate.
Reference to the use of acids as polymerization catalysts can be found in Organic Chemistry of Synthetic High Polymers, Lenz, R. W., John Wiley, New York, N.Y., 1967, pages 81-91, and Textbook of Polymer Science, 2nd ed., Billmeyer, F. W., Wiley-Interscience, New York, N.Y., 1971, pages 264-267. While the authors generally describe the use of acids, they do not disclose the use of protonic acid catalysts to reduce the formation of cyclic dimer in polymerizations involving isophthalate-based polymers.